Thread guides are used primarily in yarn cleaners. As a rule one thread guide is present ahead of and another after the detection zone of the yarn cleaner for purposes of thread guidance.
The thread guides of the state of the art usually consist of polished aluminum-ceramic plates with a V-cutout (the so-called V thread guides). They share the feature of the thread being simultaneously bounded on three different sides by surfaces present in one plane orthogonal to the direction of thread motion. Because of this simultaneous constriction at three planar points (in one radial plane), two kinds of drawbacks follow.
The first drawback is that impurities on or in the thread (particularly slubs or lumps) become stuck in the planar thread constriction. The second drawback is that the lump causes a brief jump of the thread out of the V cutout of the first thread guide and the lump continues undetected beyond the detection zone.
Considering the ever more stringent requirements presently placed on the accuracy of yarn cleaners and the presently conventional thread speeds of more than 1,000 meters/min, the above drawbacks become increasingly intolerable.